Diesters of mixed aromatic dibasic acids



2,936,320 DIESTERS F MIXED AROMATIC DIBASIC ACIDS George J. Benoit, Jr.,San Anselmo, Calif., assignor :to California Research Corporation, SanFrancisco, Calif., a corporation of Delaware No Drawing; ApplicationJune 24, 1957 Serial No. 667,716

4 Claims. (Cl. 260-475) This invention relates to novel diesters ofmixed aromatic dibasic acids. More particularly, the invention isconcerned with a new class of dialkyl diesters of mixtures ofisophthalic acid and terephthalic acidwhichare excellent synthetic oils.

Synthetic oils are important as lubricants in a variety of applicationswhere there are certain specific requirements not met by the more commonlubricating oils such as mineral oils. Applications of this type includethe lubrication of instruments, weapons, optical equipment and aircraftgas turbines of the prop and jet types.

In general, a combination of special properties is desirable insynthetic oils for applications of the above type- Fluidity at lowtemperatures and low volatility high temperatures are importantcharacteristics. Similarly, good viscosity-temperature characteristicsare desirable since properlubrication in most applications requires thatthe lubricant be adaptable to changes in temperature without extremevariations in viscosity.

The most critical characteristic of synthetic lubricating oils in theimportant application to aircraft gas turbines is the resistance of theoil to thermal degradation at high temperatures. Aircraft gas turbinesare being operated at increasingly higher temperatures where theformation of coke, due to the thermal decomposition and deterioration ofthe lubricant, is a major problem. The proper functioning of thelubrication system is impeded by the formation of deposits on hearingcages and'housings and the plugging of oil filters.

It has now been found that a superior new class of diesters of mixedaromatic dibasic acids, namely, the dialkyl diesters of mixtures ofisophthalic acid and terephthalic acid in which the alkyl esterifyinggroups are derived from a mixture of aliphatic alcohols of from 5 to 13carbon atoms each, are excellent synthetic oils having specialproperties which make them unusually desirable as synthetic lubricants.

The superior new class of dialkyl diesters of mixtures of isophthalicacid and terephthalic acid, according to the present invention asdescribed above, possesses a desirably low volatility at hightemperatures and remains fluid at low temperatures. Theirviscosity-temperature characteristics are superior to other isomericforms of diesters of aromatic dibasic acids.

The most important property of the novel dialkyl diesters of mixtures ofisophthalic acid and terephthalic acid of this invention is theirthermal stability at high temperatures which make them unusuallydesirable as base lubricants for aircraft gas turbine lubricating oilcompositions. The unusual new synthetic oils, according to the presentinvention, are characterized by a surprisingly low coke formation whensubjected to high temperatures for extended periods of time. They aresurprisingly superior in this respect to other synthetic oils commonlyemployed heretofore in high temperature 21,936,329 Patented May 1960applications such as the lubrication of aircraft gas turbines.

The dialkyl diesters of mixtures of isophthalic acid andterephthalicacid in accordance with this invention, areillustrated bythe following general formula:

l-OR:

atoms and R is another alkyl group of from 5 to 13 carbon atoms whichmay be the same as or different from R Preferably, the R and R alkylgroups are derived by esterifying the mixture of isophthalic acid andterephthalic acid with a mixture of aliphatic alcohols of from 5 to 13carbon atoms each. Mixtures of from 10 to 90% by weight of each alcoholand from 2 to 5 different kinds are particularly suitable. Still morepreferred are esterifying alcohols and alkyl groups derived therefromwithin the foregoing description which are mixed, branched chainalcohols of from 6 to 10 carbon'atoms with an average of 8 carbon atomseach. Isophthalate and terephthalate mixtures derived from such alcoholsare characterized by low volatility and at the same time have lesstendency to crystallize at low temperatures.

The mixtures of isophthalic acid and terephthalic acid from which thedialkyl diesters of this invention are derived preferably contain from50 to 99% by weight of isophthalic acid and from 1 to 50% by weight ofterephthalic acid. Such proportions occur in the usual mixtures of thetwo acids derived from isomeric xylenes from common sources such aspetroleum fractions or coal tar distillates. Particularly suitablemixtures within the aforementioned proportions are those containingabout by weight isophthalic acid and 15% by weight terephthalic acid.

Examples of dialkyl diesters of mixed isophthalic acid and terephthalicacid, in accordance with this invention, include n-hexyl 2-ethylbutylisophthalate and terephthalate having an isophthalate to terephthalateweight ratio of 25 to 75, 2-ethylhexyl isooctyl isophthalate andterephthalate derived from a 50-50 mixture of isophthalic acid andterephthalic acid, Z-ethylhexyl isooctyl diester of a mixture of 85 byweight of isophthalic acid and 15% by weight terephthalic acid,2-ethylhexyl isooctyl n-octyl mixed diester of 85% by weight isophthalicacid and 15% by weight terephthalic acid, di-Z-ethylhexyl diester of amixture of 85% by weight of isophthalic acid and 15% by weightterephthalic acid, namyl n-hexyl isooctyl mixed diester of 85:15isophthalic acid and terephthalic acid, Oxo tridecyl diester of 85:15isophthalic acid and terephthalic acid, etc.

The dialkyl diesters of isophthalic acid and terephthalic acid areconveniently prepared by the reaction of the desired aliphatic alcoholsof from 5 to 13 carbon atoms each with the mixture of acids underordinary esterification conditions. Conventional esterificationcatalysts such as p-toluene sulfonic acid and heating are desirable toaccelerate the reaction.

The esterification of the isophthalic acid and terephthalic acid withaliphatic alcohols, as described above, results in dialkyl diesters ofthree different types for the This fact is illus- Mixtures of thedialkyl diesters of the above types in which the R and R alkyl groupsare different from one another generally have superior physicalproperties compared to the diesters in which the alkyl groups are alike.

The following examples will serve as further illustraproduct isdecolorized with carbon and topped by distilling at 0.1 mm. mercurypressure to a maximum pot temperature of 200 C. It has an averagesaponification number of 294 and its equivalent weight estimated fromanalysis is about 188 compared to a theoretical of 195. The product isthen distilled at 0.1 mm. of mercury pres sure to a maximum pottemperature of 225 C. The distilled material is clear in appearance andpours slowly at about -55 F.

The physical characteristics of representative examples of the superiornew dialkyl diesters of mixtures of isophthalic acid and terephthalicacid of this invention are given in the following table. Also included,for the purpose of comparison, are esters of phthalic acid which istions of the dialkyl diesters of isophthalic acid and terthe orthoisomer of isophthalic acid and terephthalic acid.

Table 1 V Viscosity, Centistokes Approximate Acid Alkyl Groups Pouggoint,

' -40 F. 100 F 210 F.

'Phthallc 2-Ethylhexyl 100,000 extra--- 29. 6 4. Cloud at 30. Isooetyl65,000 extr-... 27.2 4. 14 Cloud at --20.

2-Etl1ylhexyl-lsooetyl 77,0 33.5 4.86 -55. 85IPA15TPA2-Ethylhexyl-Isooctyl-n-Octyl 48,000 extr 28.5 4.55 -so.

ephthalic acid mixtures, according to this invention. Unless otherwisespecified, the proportions are given on a weight basis.

EXAMPLE I This example shows the preparation of mixed isooctyl, n-octyl,2-ethylhexyl ester of a mixture of 85% by weight isophthalic acid and15% by weight terephthalic acid.

322 g. of 85:15 isophthalic acid and terephthalic acid mixture, 546 g.isooctyl alcohol (75%) 82 g. 2-ethylhexanol (10%) and 109 g. n-octanol(15%) are mixed in a reaction vessel equipped with stirrer, thermometer,water trap and reflux condenser. 5.3 g. sodium bisulfate is added to themixture. The mixture is heated under reflux conditions for about 16hours to a maximum pot temperature of about 215 C. under a blanket ofnitrogen. 75 ml. water is evolved during this time. This preparation iscarried out in duplicate to provide additional product.

The combined products obtained above are washed twice with diluteaqueous sodium carbonate solution and then water until neutral. Theproducts are then treated with g. decolorizing carbon and topped bydistilling at 0.1 mm. mercury pressure to a maximum pot temperature of200 C. to remove unreacted materials. 1,428 g. of bottoms product isobtained. This product is filtered through a column filled with 300 g.of clay to give 1,058 g. of mixed isooctyl, n-octyl, 2-ethylhexyl esterof 85: 15 isoand terephthalic acid which has a clear appearance andpours slowly at about -60 F.

EXAMPLE II The preparation of mixed isooctyl, Z-ethylhexyl diester of amixture of 85% by weight isophthalic acid and 15 by weight terephthalicacid is shown in this example.

332 g. of 85:15 isophthalic acid and terephthalic acid In the abovetable the abbreviation extr. indicates that the viscosity was obtainedby extrapolation of the viscosity-temperature curve for the fluid.

From the physical properties of the phthalate esters in the above table,it is readily'observed that the mixed dialkyl diesters of isophthalicacid and terephthalic acid are decidedly superior to the correspondingesters of phthalic acid as base lubricants. The viscosity-temperaturecharacteristics are much better, thus'providing more effectivelubrication over wide temperature ranges. Another advantage of the mixeddialkyl isophthalates and terephthalates lies in their low pour points.

A further illustration of the superior new dialkyl diesters ofisophthalic acid and terephthalic acid mixtures of the invention isobtained in an evaluation of the effectiveness of the phthalic esters asaircraft gas turbine lubricants by a number of standard tests on thephthalic ester base lubricants compounded with conventional additives.In these tests the comparison of the results is facilitated by theemployment of identically formulated I compositions as follows:

As mentioned, the tests employed in the evaluations are standard testsknown generally to the lubricating art by their descriptions in thetable which follows.

The standard coking test involves maintaining the test oil in a bath ata constant level and splashing it against an overhead inclined plate atthe temperature indicated for eight hours. The splashing is effected bysteel wires on a wheel rotated at a speed of 1,050 rpm. as it ispartially immersed in the oil. The coking value is the weight of thedeposit formed on the underside of the plate during the test period. Thedescription of this coking test is found in gas turbine oilspecifications MIL- L-78 08C and MIL-L-25336.

The evaporation characteristics of the compositions are determined byplacing them in a bomb through. which air is passed at a given rate overthe fluid. The weight losses over the period of time at the temperatureindicated are reported in terms of percent by weight. This test is thestandard ASTM evaporation test D972-51T.

The oxidation and corrosion test involves joining the five metalspecimens in the form of a pentagon and immersing them in the oil to betested. Oxygen is bubbled through the oil at the temperature and for thetime indicated and the change in weight of each of the metal specimensis determined.

For the purpose of comparison, the following table also givesillustrated test results on compositions containing base lubricantsother than the mixed dialkyl diesters of isophthalic acid andterephthalic acid according to this invention. For example,di(Z-ethylhexyl) sebacate is included as illustrative of one of the morecommonly used synthetic oil base lubricants. The separate phthalateisomers are also given for comparative purposes.

factorily low level and there is little sludge formed in thecompositions.

The novel diesters of mixed isophthalic acid and terephthalic acid haveother uses in addition to their application as synthetic oils. Such usesinclude their employment as plasticizers in compositions where lowfreezing points are desirable.

I claim:

1. Dialkyl diesters of mixtures of isophthalic acid and terephthalicacid in which the alkyl esterifying groups are derived from a mixture offrom 2 to 5 aliphatic alcohols of 8 carbon atoms each said mixturecontaining from 10 to 90% by weight of each alcohol and the mixtures ofacid contain from to 99% by weight of isophthalic acid and from 1 to 50%by weight of terephthalic acid.

2. Dialkyl diesters of a mixture of 85% by weight of Table 2 Ester Type:Di-z-ethyl- Isooctyl Alkyl Group z-Ethylhexyl, Isooetyl hexyl n-octyl 2-ethylhexyl Iso: Ter- Iso: Ter- Acid Group Iso-Phthai- 'Ierephephthalatesebacate ephthalate ate thalate Ratio, (Reference) Ratio,

Properties:

Flash, F 450 Fire, F 610 Viscosity at 210 F., cs 3.38 4. 56 Viscosity at100 F., cs 32. 74 31. 54 32. 9 13. 1 28. 6 Viscosity at 40 F., cs135,000 104,000 92, 000 1, 700 64, 500 Neutralization N o 0.31 0. 0. 520. 90 0. 40 Oxidation and Corrosion, 72 hours at 347 F.,

Metal Wt. Change, MgJcmJ:

Magnesium +0. 06 Steel +0. 03 +0. ()6 +0.05 +0. 09 Percent change in 100F. vis +4. 66 Change in Neutralization N o +0.1 Percent pentaneinsolubles 0. 42 Coking, 8 hours at 600 F., mg Coking, 8 hours at 700F., mg 904 Used Oil from Sump:

Percent change in 100 F. vis +11. 7 Change in Neutralization No +3.0Percent pentane insolubles 0. 24 Evaporation, 615 hours at 400 F.,Percent 15 1 Coke value equivalent to an 8-hour test. Test terminatedbeforeS hours.

The above test results show that the mixed dialkyl diesters ofisophthalic acid andterephthalic acid are outstanding synthetic oil basefluids for aircraft gas turbine lubricating oils. They possess excellentthermal stability at high temperatures as indicated by their low cokeformation. In this very important characteristic for gas turbinelubricants, it can be seen at once from the above results that the mixeddialkyl diesters of isophthalic acid and terephthalic acid givelubricant compositions which are decidedly superior to similarcompositions containing the widely used di(Z-ethylhexyl) sebacate asbase lubricant. The coking characteristics of the mixed isophthalatesand terephthalates of the invention are also surprisingly superior tothe separate isomeric phthalates.

The above table also shows that the mixed isophthalate and terephthalatebase gas turbine lubricant compositions of the invention possessexcellent volatility characteristics which prevent undue consumption ofthe lubricant compositions at high altitudes and high temperatures. Themixed isophthalates and terephthalates are also fluid at unusually lowtemperatures and have good viscosity tem perature characteristics whichmake them suitable as lubricants throughout wide temperature ranges.

The compositions of the invention are also shown to be unusuallyresistant to oxidation and formation of corrosive products at hightemperatures. The corrosion of all types of metals in the tests ismaintained at a satisisophthalic acid and 15% by weight of terephthalicacid in which the alkyl esterifying groups are derived from a mixture offrom 2 to 5 aliphatic alcohols of 8 carbon atoms each, said mixture ofalcohols containing from 10 to 90% by weight of each alcohol.

3. 2-ethylhexyl isooctyl diester of a mixture of by weight isophthalicacid and 15% by weight terephthalic acid in which the 2-ethylhexyl andisooctyl esterifying groups are derived from a mixture consisting of 25%by weight of '2-ethylhexyl alcohol and 75% by weight of isooctylalcohol.

4. Mixed isooctyl, n-octyl and 2-ethylhexy1 diesters of a mixture of 85%by weight isophthalic acid and 15% by weight terephthalic acid in whichthe esterifying groups are derived from a mixture consisting of 75 byWeight of isooctyl alcohol, 15% by weight of n-octyl alcohol and 10% byweight of 2-ethylhexyl alcohol.

References Cited in the file of this patent UNITED STATES PATENTS2,610,205 Trigg et al. Sept. 9, 1952 2,628,207 Smith et al. Feb. 10,1953 2,650,908 Beears Sept. 1, 1953 2,653,165 Levine Sept. 22, 19532,656,377 Pino Oct. 20, 1953 2,802,860 Stack et al. Aug. 13, 19572,809,208 Miller Oct. 8. 1957

1. DIALKYL DIESTERS OF MIXTURES OF ISOPHTHALIC ACID AND TEREPHTHALICACID IN WHICH THE ALKYL ESTERIFYING GROUPS ARE DERIVED FROM A MIXTURE OFFROM 2 TO 5 ALIPHATIC ALCOHOLS OF 8 CARBON ATOMS EACH SAID MIXTURECONTAINING FROM 10 TO 90% BY WEIGHT OF EACH ALCOHOL AND THE MIXTURES OFACID CONTAIN FROM 50 TO 99% BY WEIGHT OF ISOPHTHALIC ACID AND FROM 1 TO50% BY WEIGHT OF TEREPHTHALIC ACID.